Systems, methods, and devices for interference mitigation in wireless networks

ABSTRACT

Example systems, methods, and devices for mitigating interference in wireless networks are discussed. One example method includes the operations of passing channel frequency offsets of a plurality of LTF symbols on a plurality of subcarriers through a high pass frequency band, encoding the plurality of LTF symbols with a plurality of LTF sequences across frequency, and encoding the LTF symbols in time and/or frequency. Another example includes the operations of receiving a plurality of LTF symbols on a plurality of subcarriers for channel estimation of one or more streams, removing the encoding across time, removing the encoding across frequency, and removing the LTF sequence(s), and passing the modified LTF symbols through a smoothing filter, for example, a low pass filter for removing the interference due to CFOs. Methods, apparatus, and systems described herein can be applied to 802.11ax or any other wireless standard.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/081,996 filed on Nov. 19, 2014, the entire contents of which areincorporated herein by reference.

TECHNICAL FIELD

Embodiments described herein generally relate to wireless networks and,more particularly, to mitigating interference in a wireless network.

BACKGROUND

Wi-Fi network performance is an important factor in environments withhigh numbers of users, such as hotspots in public venues. Efficient useof available spectrum and better management of interferences in a Wi-Fienvironment may improve Wi-Fi performance. In order to address the issueof increasing bandwidth requirements that are demanded for wirelesscommunications systems, different schemes may be employed to allowmultiple user devices to communicate with a single access point bysharing the channel resources while achieving high data throughputs.Multiple Input or Multiple Output (MIMO) technology represents one suchscheme that has emerged for wireless communication systems. MIMOtechnology has been adopted in several emerging wireless communicationsstandards such as the Institute of Electrical and Electronics Engineers(IEEE) 802.11 standard.

In addition, a next generation wireless local area network (WLAN), IEEE802.11ax or High-Efficiency WLAN (HEW), is under development. Uplinkmultiuser multiple-input multiple-output (UL MU-MIMO) and OrthogonalFrequency-Division Multiple Access (OFDMA) are two features included inthat standard. However, modulation and coding schemes (MCS) 7-9 cannotbe reliably supported in UL MU-MIMO using existing systems and methods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a network diagram illustrating an example wireless networkenvironment, according to one or more example embodiments;

FIG. 2 illustrates an example wireless network environment including ULMU-MIMO with channel frequency offsets from a plurality of users,according to one or more example embodiments;

FIG. 3 illustrates example methods for encoding data streams usingP-matrices in a wireless network, according to one or more exampleembodiments;

FIG. 4 illustrates interference characteristics of a channel frequencyoffset across a frequency domain in a wireless network, according to oneor more example embodiments;

FIG. 5 illustrates horizontally and vertically cyclically shiftedorthogonal codes for a plurality of subcarriers, according to one ormore example embodiments;

FIG. 6 illustrates example methods for encoding data streams usingP-matrices across frequency and time domains, according to one or moreexample embodiments;

FIG. 7 illustrates example operations in a method for use in systems anddevices, according to one or more example embodiments;

FIG. 8 illustrates example operations in a method for use in systems anddevices, according to one or more example embodiments;

FIG. 9 illustrates example operations in a method for use in systems anddevices, according to one or more example embodiments;

FIG. 10 illustrates a functional diagram of an example communicationstation or example access point, according to one or more exampleembodiments; and

FIG. 11 shows a block diagram of an example of a machine upon which anyof the techniques (e.g., methods) according to one or more exampleembodiments discussed herein may be performed.

DETAILED DESCRIPTION

Example embodiments described herein provide systems, methods, anddevices, for mitigating interference in various Wi-Fi networks,including but not limited to IEEE 802.11ax.

The terms “communication station”, “station”, “handheld device”, “mobiledevice”, “wireless device” and “user equipment” (UE), as used herein,refer to a wireless communication device such as a cellular telephone,smartphone, tablet, netbook, wireless terminal, laptop computer, awearable computer device, a femtocell, High Data Rate (HDR) subscriberstation, access point, access terminal, or other personal communicationsystem (PCS) device. The device may be either mobile or stationary.

The term “access point” (AP) as used herein may be a fixed station. Anaccess point may also be referred to as an access node, a base stationor some other similar terminology known in the art. An access terminalmay also be called a mobile station, a user equipment (UE), a wirelesscommunication device or some other similar terminology known in the art.Embodiments disclosed herein generally pertain to wireless networks.Some embodiments can relate to wireless networks that operate inaccordance with one of the IEEE 802.11 standards including but notlimited to the IEEE 802.11ax standard.

FIG. 1 is a network diagram illustrating an example wireless networkenvironment, according to some example embodiments. Wireless network 100can include one or more communication stations (STAs) 104 and one ormore access points (APs) 102, which may be connected via a wireless link106 and may communicate in accordance with any wireless communicationstandard, including but not limited to the IEEE 802.11ax standard. Thecommunication stations 104 may be mobile devices that may benon-stationary, and the APs 102 may be stationary and have fixedlocations.

In accordance with some IEEE 802.11ax (High-Efficiency WLAN (HEW))embodiments, an access point may operate as a master station which maybe arranged to contend for a wireless medium (e.g., during a contentionperiod) to receive exclusive control of the medium for an HEW controlperiod (i.e., a transmission opportunity (TXOP)). The master station maytransmit an HEW master-sync transmission at the beginning of the HEWcontrol period. During the HEW control period, HEW stations maycommunicate with the master station in accordance with a non-contentionbased multiple access technique. This is unlike conventional Wi-Ficommunications in which devices communicate in accordance with acontention-based communication technique, rather than a multiple accesstechnique. During the HEW control period, the master station maycommunicate with HEW stations using one or more HEW frames. Furthermore,during the HEW control period, legacy stations refrain fromcommunicating. In some embodiments, the master-sync transmission may bereferred to as an HEW control and schedule transmission.

In some embodiments, the multiple-access technique used during the HEWcontrol period may be a scheduled orthogonal frequency division multipleaccess (OFDMA) technique, although this is not a requirement. In otherembodiments, the multiple access technique may be a time-divisionmultiple access (TDMA) technique or a frequency division multiple access(FDMA) technique. In certain embodiments, the multiple access techniquemay be a space-division multiple access (SDMA) technique also calledmultiuser multiple-input multiple-output (MU-MIMO).

The master station may also communicate with legacy stations inaccordance with legacy IEEE 802.11 communication techniques. In someembodiments, the master station may also be configurable communicatewith HEW stations outside the HEW control period in accordance withlegacy IEEE 802.11 communication techniques, although this is not arequirement.

In other embodiments, the links of an HEW frame may be configurable tohave the same bandwidth and the bandwidth may be one of 20 MHz, 40 MHz,or 80 MHz contiguous bandwidths or an 80+80 MHz (160 MHz) non-contiguousbandwidth. In certain embodiments, a 320 MHz contiguous bandwidth may beused. In other embodiments, bandwidths of 5 MHz and/or 10 MHz may alsobe used. In these embodiments, each link of an HEW frame may beconfigured for transmitting a number of spatial streams.

A feature of DensiFi (e.g., a possible next generation wireless localarea network (WLAN), IEEE 802.11ax or High-Efficiency WLAN (HEW)) is achannel training signal design for uplink multiuser MIMO (UL MU-MIMO).While one aspect reuses the P-matrix structure in 802.11n/ac, and otheraspect uses a new tone-interleaved structure. A drawback of the P-matrixstructure, however, is that the P-matrix may be vulnerable to channelfrequency offsets (CFOs) in a multiuser MIMO (MU-MIMO) as illustrated inFIG. 2, for example. Access point (AP) 202 may communicate with aplurality of communication stations (STAs) 204 in an example Wi-Finetwork 200, according to one or more example embodiments. P-matrixdecoding assumes little or no CFOs across long training field (LTF)symbols over time and relies on the orthogonal codes defined in theP-matrix to remove the potential interferences from the other spatialstreams. However, the CFOs in uplink MU-MIMO are not negligible for highmodulation orders, and different users may have different CFOs.Accordingly, the access point (AP) receiver has to estimate the CFO ofeach user in the presence of interference from other users. This couldbe challenging in some instances. CFOs from different users can destroythe orthogonality of the P-matrix and cause interferences with eachother. Modulation and coding schemes (MCS) 7-9 cannot be reliablysupported in uplink MU-MIMO using the existing systems andmethodologies.

The systems, methods, and devices described in the present disclosureprovide an enhancement to the P-matrix structure such that CFO of eachuser can be estimated and the interference due to other users' CFOs canbe mitigated. The following description and the drawings sufficientlyillustrate specific embodiments to enable those skilled in the art topractice them. Other embodiments may incorporate structural, logical,electrical, process, and other changes. Portions and features of someembodiments may be included in, or substituted for, those of otherembodiments. Details of one or more implementations are set forth in theaccompanying drawings and in the description below. Further embodiments,features, and aspects will become apparent from the description, thedrawings, and the claims. Embodiments set forth in the claims encompassall available equivalents of those claims.

Referring now to FIG. 2, a plurality of streams (e.g., signals) areillustrated, each having a different channel frequency offset. Streamstypically comprise a sequence of one or more LTF symbols 206 and whichare received by a receiver, such as access point(s) 202, from differentchannels (and/or subbands of channels) associated with different usersand/or stations 204 (e.g., user devices or other devices such as acontent-providing platform, a control system, a network distributionengine, wireless stations, and/or the like) in an uplink MU-MIMO. LTFsymbols are then encoded into one or more streams (e.g., channelresponses, signals at a particular channel frequency, an audio signal, avideo signal, a data signal, and/or the like) by different users usingP-matrix codes in an uplink (e.g., upon transmittal of a stream from auser device to an access point, for example).

In some embodiments, different LTF sequences and/or different P-matricesmay be constructed and used for encoding and/or decoding differentstreams in uplink. FIG. 2 depicts an uplink process between a pluralityof stations 204 (e.g., STA, user devices, and/or the like) and an accesspoint 202. In other instances, a P-matrix-encoded LTF may be reused foruplink for different streams with various devices. In this way, the sameP-matrix may be used for both encoding and decoding different streamswith sequences of LTF symbols. Encoding and/or decoding can occur duringuplink or downlink transmissions at an access point 202 and/or a station204 (or other device). However, in such instances, a P-matrix-encodedLTF in an uplink MU-MIMO may have the same or different channelfrequency offsets (CFOs) for each stream of channels and/or subbands ofchannels associated with different users and/or devices, as shown inFIG. 2.

As such, it may be difficult to estimate a CFO of each user and/or userstations 204 to accurately identify a channel frequency of each channeldue to interferences from various devices communicating on the samechannels and/or subbands of channels. For example, having different CFOson the same channel may introduce unwanted interferences among theuplink MU-MIMO streams and creates issues during processing (e.g.,encoding and/or decoding) of streams. Furthermore, because the CFOs ofthe uplink devices may be calculated by via measuring (e.g.,determining) a phase change across time of each device's signalstransmitting streams of LTF symbol sequences, any introducedinterference may skew estimations of CFOs. Therefore, in uplink MU-MIMO,CFO correction processes may be unable to utilize LTFs and may bedelayed until pilots (e.g., known training signals on known subcarriersfor channel frequency tracking or channel variation tracking, and/or thelike) in data transmissions are received.

For ease of receiver implementation at the access point 202, LTF symbolsequences of some or all streams are terminated at the same time in theuplink by stations 204 and in the downlink by the access point 202. Byterminating some or all streams of a channel and/or subbands of achannel at the same time in the uplink or in the downlink, the accesspoint 202 assigns the same number of LTF symbols to each stream. In thismanner, the number of LTF symbols for each channel and/or each subbandof a channel is determined by the channel and/or subband of a channelwith the maximum number of streams. The access point 202 (and/or aprocessing device of the access point 202) is configured to determine anumber of streams in each channel and/or subband of a channel. TheP-matrix sizes may be 2×2, 4×4, 6×6, or 8×8, however, the number of LTFsymbols is rounded to the least P-matrix size e.g. 2, 4, 6, or 8 that isgreater than or equal to the maximum number of streams. Therefore, eachchannel and/or subband of a channel will have the same number of LTFsymbols in its LTF sequence used for encoding and/or decodingP-matrices.

For the channels and/or subbands of a channel determined to have anumber streams less than the maximum number of streams, extra LTFsymbols (e.g, additional LTF symbols that aren't required fortransmitting signals (e.g., streams) on the channels and/or subbands ofa channel determined to have a number streams less than the maximumnumber of streams) can be added to the LTF symbols in the LTF symbolsequence that is required for transmitting signals (e.g., streams) oneach channel and/or subband of a channel. For example, the first fewrows of a larger P-matrix as in 802.11n/ac may be utilized to replace asmaller P-matrix that is required when transmitting signals on eachchannel determined to have a number streams less than the maximum numberof streams. However, this method may not allow the receiver to decodethe channel response using part of LTF symbols.

For CFO estimation (e.g., determination) as described herein, thereceiver (e.g., a device, a user device, a station 204, and/or the like)can observe channel responses (e.g., signals and/or streams includingLTF symbol sequences) separated in time such that a difference in phasecan be detected and then used for CFO estimation of the channel. Insteadof using the first few rows (or columns) of a larger P-matrix forencoding and/or decoding of LTF symbols into a stream, a P-matrix assmall as possible may be selected such that the receiver (e.g., anaccess point 202 and/or a user device or station 204) can decode thechannel response with the smallest number of LTF symbols. For example,if the number of required LTF symbols for the whole channel is four anda subband of a channel has two streams, then a selected P-matrix usedfor encoding and/or decoding may be a 2×2 P-matrix instead of a legacy4×4 P-matrix. Typically, a P-matrix is a square orthogonal matrix,wherein all rows of the P-matrix are orthogonal with each other, and allcolumns are also orthogonal with each other. However, while a P-matrixmay be desirable, any orthogonal matrix can be used in place of aP-matrix.

In addition and as discussed above, additional LTF symbols may be padded(e.g., added into one or more columns of a selected P-matrix) because acode length of a transmission is shorter than a number of LTF symbolsrequired for transmitting a stream. The code length of a transmissionmay be determined by the access point 202 and may correspond to a numberof LTF symbols required for transmission of a stream. For example, asubband of a channel that includes four streams may require four LTFsymbols for transmission, whereas a second subband of the same channelmay include only two streams and therefore may require only two LTFsymbols for transmission. In this manner, the access point 202determines that the second subband has a shorter code length than thefirst subband based on a comparison of a number of required LTF symbolsfor each subband. The access point 202 then may determine that twoadditional LTF symbols are to be padded into the second subbandtransmission so that the number of LTF symbols of the first and secondsubbands (and potentially all other subbands in the same channel) arethe same. The access point 202 may further determine a P-matrix sizerequired for transmission using a channel and/or subband of a channelbased on a required number of LTF symbols. As an alternative for thisexample, the access point 202 may determine that 4×4 P-matrix may beused. The first subband uses all four rows of the 4×4 P-matrix for thefour streams, respectively; and the second subband uses only the firsttwo rows for the same 4×4 P-matrix for the two streams, respectively.

In some embodiments, a sequence of LTF symbols may be utilized forencoding and/or decoding a transmission, such as a channel response.Sequences of LTF symbols may be included at the beginning and/or the endof a transmission in a preamble or data header. Alternatively, sequencesof LTF symbols may be embedded into a transmission data signal usingdigital watermarking and/or other encoding techniques common in the art.Typically, LTF symbols are encoded into a transmission data signal usinga P-matrix. Again, using the smallest P-matrix possible based on adetermined number of streams in a channel and/or subband of a channel,additional LTF symbols are padded such that a channel response (e.g., astream, a transmission of data, content, and/or the like) can be encodedand/or decoded with the fewest LTF symbols at the end and/or beginningof a LTF symbol sequence. Finally, for maximizing time separationbetween channel observations (e.g., receipt of channel responses), anumber of P-matrix columns is determined and selected by the accesspoint 202 such that the beginning and/or the end of an LTF symbolsequence has a complete set of LTF symbols sufficient to encode and/ordecode the channel response after its transmission. In this manner, theaccess point 202 and stations 204 are enabled to communicate with oneanother using a common sequence of LTF symbols.

Referring now to FIG. 3, illustrating LTF structure 300 in 802.11n/acstandard wireless communication, the number of streams of a channeland/or a subband of a channel can be smaller than a number of LTFsymbols required for the channel (and, if applicable, all of itsincluded subbands). For example, if the number of streams is three, thenumber of LTF symbols can be four. In this case, the P-matrix for fourstreams is used and the first three rows are used to fill the P-matrixwith LTF symbols, thereby forming a 3×4 code matrix, as shown in FIG. 3.The 1 and −1 coefficients in FIG. 3 form a 4×4 P-matrix 304, asillustrated below:

$\begin{bmatrix}1 & {- 1} & 1 & 1 \\1 & 1 & {- 1} & 1 \\1 & 1 & 1 & {- 1} \\{- 1} & 1 & 1 & 1\end{bmatrix}\quad$

A first channel and/or a subband of a channel 302 may include just onestream, a second channel and/or a subband of a channel 304 may includetwo streams, a third channel and/or a subband of a channel 306 mayinclude three streams, and a fourth channel and/or a subband of achannel 308 may include four streams, according to one or more exampleembodiments. Each of the streams 302-308 may include one or more LTFsymbols 310, as illustrated in FIG. 3. In some embodiments, an accesspoint may select and/or determine a number of LTF symbols for allsubbands and/or subchannels. A P-matrix size is then determined based onthe selected number of LTF symbols. Typically, the determined P-matrixsize is the same as the selected number of LTF symbols (e.g., 2, 4, 8,and/or the like). For example, if 4 LTF symbols are required fortransmission on each subband and/or subchannel, then the P-matrix sizemay be 4×4. In this manner, a P-matrix of a common size may be utilizedfor all subbands and/or subchannels. Furthermore, if a subband and/orsubchannel has ‘N’ streams, then the first N rows of the P-matrix may beused.

In some embodiments, CFO estimation includes determining a timedifference between clocks of a transmitter (e.g., an access point) and areceiver (e.g., a user device). For example, the receiver may obtain atleast two observations (e.g., data points) about a channel response(e.g., a stream) of a channel and/or a subband of a channel at twodifferent times. First, a stream (e.g., a channel response, a datasignal, a transmission, a pulse, a ping, and/or the like) is generatedand encoded with LTF symbols by the transmitter. The stream is thentransmitted by the transmitter to the receiver. During reception of thestream (e.g., upon receiving a first LTF symbol at the beginning of anLTF symbol sequence), the receiver measures (e.g., determines) a firstphase of the channel and/or subband on which the stream is transmittedand/or received. The measured (e.g., determined) first phase and a firsttime-stamp of the stream transmission can be recorded (e.g., stored).Next, a subsequent training signal (e.g., an additional LTF symbol ofthe transmitted stream) is received by the receiver. Upon receipt of theadditional training signal of the stream at the receiver (e.g., uponreceiving a second LTF symbol of an LTF symbol sequence), the receivermeasures (e.g., determines) a second phase of the channel and/or subbandon which the signal is transmitted and/or received. The measured (e.g.,determined) second phase and a second time-stamp can be recorded (e.g.,stored). The receiver may also measure (e.g., determine) a third phaseof the channel and/or subband on which the signal is transmitted and/orreceived upon receipt of a last LTF symbol at the end of an LTF symbolsequence. The measured (e.g., determined) third phase and a thirdtime-stamp can be recorded (e.g., stored).

Alternatively, a plurality of streams may be transmitted and/or receivedat different times. The receiver and/or the transmitter may be thedevice responsible for obtaining a first, second, third, and anysubsequent phase measurement and/or time-stamp of a stream. In someembodiments, creating a larger difference in time between obtainingfirst and second observations (and any subsequent observations) about achannel response may assist in providing more accurate measurements ofphase difference of a channel and/or a subband of a channel, therebyresulting in a more accurate calculated CFO. For example, for a givenCFO, a larger time difference typically corresponds to a larger phasedifference, which provides a more accurate CFO estimation. Additionally,more than two or three observations may be obtained, and multiple phasemeasurements may be taken and processed.

Typically, a channel frequency offset (CFO) is associated with areceiver and/or a transmitter device that has a clock difference betweenanother device (e.g., a transmitter and/or a receiver). In someembodiments, a single subband and/or subchannel may facilitatecommunication between multiple receiving devices (in downlink multi-userMIMO, for example) and/or between multiple transmitting devices (inuplink multi-user MIMO, for example). A CFO is typically calculatedbetween each pair of receiving devices and transmitting devices.

A CFO of the channel and/or subband of the channel can be estimated fromthe phase difference between the channel responses. So, the first phaseand the second phase are processed by a computing device to calculate aphase difference between the first phase and the second phase. A channelfrequency offset (CFO) is then calculated based on the calculated phasedifference divided by a calculated time difference corresponding to ameasured difference between two phase measurements. In this manner, CFOestimation for a channel and/or subband of a channel whose number ofstreams is less than the number of LTF symbols may be determined usingthe systems, methods, and apparatuses described herein.

In some embodiments of the present disclosure, the number of LTF symbolsof each subband of an entire channel may be determined by a subband withthe maximum number of streams. Let n1×n1, n2×n2, . . . , nP×nP be theP-matrix sizes defined in the new standard equal to or greater than themaximum number of streams among all subbands. For example, in legacy802.11ac, defined P-matrix sizes are 2×2, 4×4, 6×6, and 8×8. For maximumseven streams, a 8×8 P-matrix can be used, and eight LTF symbols areneeded.

In certain embodiments of the present disclosure, a LTF symbol number ofa particular channel and/or a subband of a channel may be indicated byan access point or a mobile station (e.g., a user device, a transmitter,a receiver, and/or the like). In this way, the same LTF symbol sequencemay be assigned to a plurality of signals, thereby ensuring morereliable channel encoding, decoding, and/or CFO estimation. For example,a transmitter's clock may fluctuate due to a switch from receive mode totransmit mode (e.g., when switching between an uplink mode and adownlink mode). In some embodiments, a clock settling time of somedevices may be the same or different than others. Therefore, additionalLTF symbols may be used for accommodating devices with a slower settlingto thereby increase accuracy of any CFO estimations.

For each channel and/or subband of a channel, the smallest code matrix Cfor encoding and/or decoding a LTF symbol sequence may be determined.For example, N may be a number of streams of a channel and/or a subbandof a channel. If an N×N P-matrix is defined in a wireless standard, thecode matrix C can be an N×N P-matrix. Otherwise, suppose M is thesmallest number that has a defined M×M P-matrix in a wireless standard,where M≧N. N rows (or columns) of the M×M P-matrix can be used to formthe code matrix C. For example, if an N×M P-matrix is used, the codematrix C can defined by the first N rows (or columns) of the defined M×MP-matrix. In another example, if the number of streams is two and thenumber of LTF symbols is four, then two defined 2×2 P-matrixes may beused for LTF symbols. In contrast, other designs may use the first tworows of the 4×4 P-matrix. If a channel and/or a subband of a channel hasa number of streams equal to three but a 3×3 P-matrix is not defined inthe standard, the first three rows of the smallest defined P-matrix thatsupports three streams may be used (e.g., 4×4 P-matrix). Using thesmallest code matrix C, as described herein, allows the receiver toobtain channel observations (and therefore calculate estimates of CFOs)using the fewest or least number of LTF symbols as allowed by wirelessstandards.

For each channel and/or a subband of a channel, after a smallest N×Mcode matrix C is determined, where N≦M, both first M symbols (e.g., afirst column of M) and the last M symbols (e.g., a last column of M) ofthe LTF symbol sequence are encoded by the code matrix C completely. Asa result, a device (e.g., a receiver, a transmitter, a user device, astation 204, and/or the like) can obtain a set of channel estimates(e.g., phase measurements, CFO estimates, and/or the like) using first MLTF symbols when first receiving and/or decoding at a beginning of a LTFsequence and another set of channel estimates using last M LTF symbolsat an end of the same LTF sequence. Note that M can be at least a numberof LTF symbols required for obtaining a set of channel estimates byinverting the N×M code matrix C. Again, a larger time separation betweenobtaining two (or more) sets of channel observations results in agreater phase difference and, therefore, more accurate CFO estimations.

In 802.11n/ac, P-matrix may be applied to long training field (LTF)symbols across time domain, as illustrated in FIG. 3, for example. Asillustrated in FIG. 3, two streams may be sent as shown in the top leftcorner, and two LTFs may be used, for example. For stream 1, two HT-LTFsymbols may be sent over two OFDM symbols multiplied by (1, −1). Forstream 2, two HT-LTF symbols may also be sent over the same two OFDMsymbols superimposed on top of stream 1's and multiplied by (1, 1) asshown inside the dashed line box in FIG. 3, for example.

Since (1, −1) and (1, 1) are orthogonal sequences, each receiver may beable to estimate the channel responses of stream 1 and stream 2 byapplying the corresponding orthogonal codes. For example, if a receiverwants to estimate the channel response of stream k for some subcarrier,then it can calculate a weighted sum of the received LTF symbols as{circumflex over (h)}(k)=pΣ _(i=1) ^(N) c _(i)*(k)s*r _(t),  (1)

where N is the number of OFDM or OFDMA symbols of the LTF, for example,the code length; ĥ(k) is the estimated channel response for thesubcarrier; p is the power normalization factor, for example 1/N; r_(i)is the received signal on the subcarrier on the i-th OFDM or OFDMAsymbol of the LTF; c_(i)(k) is the i-th entry of the orthogonal code forstream k; c_(i)*(k) is the conjugate of c_(i)(k); s* is the conjugate ofthe transmitted LTF signal the same for all streams on the subcarrier.It should be noted, however, that s may not vary over the OFDM symbolsor with index i or with stream index k and it may only vary withfrequency as in the legacy 802.11n/ac. The received signal r_(i) may bea superimposition of all K streams' signals asr _(i)=Σ_(i=1) ^(K) c _(i)(k)sh(k)+n _(i),  (2)

where h(k) is the channel response of stream k for the subcarrier; n_(i)is the noise plus co-channel interference; K is the number of streams.Substitution of (2) into (1) gives{circumflex over (h)}(k)=h(k)+p∥s∥ ²Σ_(k′≠k) h(k′)Σ_(i=1) ^(N) c_(i)*(k)c _(i)(k′)+ε(k),  (3)

where ε(k) is the noise plus co-channel interference;

p∥s∥²Σ_(k′≠k)h(k′)Σ_(i=1) ^(N)c_(i)*(k)c_(i)(k′) is the aggregatedcrosstalk from the other K−1 streams. Since the code sequences ofdifferent streams are orthogonal i.e. Σ_(i=1) ^(N)c_(i)*(k)c_(i)(k′)=0for k′≠k, there may be zero crosstalk among the streams as

$\begin{matrix}\begin{matrix}{{{s}^{2}{\sum\limits_{k^{\prime} \neq k}{{h\left( k^{\prime} \right)}{\sum\limits_{i = 1}^{N}{{c_{i}^{*}(k)}{c_{i}\left( k^{\prime} \right)}}}}}} = {{s}^{2}{\sum\limits_{k^{\prime} \neq k}{{h\left( k^{\prime} \right)} \cdot 0}}}} \\{= 0.}\end{matrix} & (4)\end{matrix}$

It should be noted, however, that the zero crosstalk in (4) relies onzero CFO. Namely, the received LTF signal s remains the same across theN LTF symbols.

However, in reality, there are residual CFOs in the transmitter clocksof the different STAs in the uplink MU-MIMO. Therefore, the received LTFsignal of the k-th stream, which is down converted by the AP's clock,varies across the LTF symbols as{tilde over (s)}(k)=e ^(jiΔω) ^(k) ^(T) s(k)  (5)

where T is the OFDM or OFDMA symbol duration; Δω_(k) is the CFO ofstream k; i is the LTF symbol index from 1 to N. The additional phaseterm e^(jiΔω) ^(k) ^(T) makes {tilde over (s)}(k) vary from one LTFsymbol to the other. As a result, the interference in (4) doesn't go tozero and it should be rewritten as∥s∥ ²Σ_(k′≠k) h(k′)Σ_(i=1) ^(N) e ^(jiΔω) ^(k′) ^(T) c _(i)*(k)c_(i)(k′)  (6)

The interference due to CFO degrades the channel estimate ĥ(k) and thusthe packet error rate performance. It is therefore desirable to reducethe interference of (6).

Turning now to FIG. 4, FIG. 4 illustrates interference characteristicsof CFOs across frequency domain. The aggregated interference 402 fromthe other streams due to the CFOs varies slowly across frequency 410 asshown in the left portion of FIG. 4. The reason could be that theunderlying channel responses of the streams vary slowly acrossfrequency, for example, they may be correlated. This is true especiallybecause beam forming is usually not applied in the uplink. Examplesystems, methods and devices disclosed may make use of the channelcorrelation to mitigate the CFO interference if the CFO can be convertedinto a high pass process 404 in frequency, for example.

Example systems, methods, and devices disclosed can introduce aperturbation in the interference such that the CFO interference is ahigh pass process 404 in contrast to the low pass process of the channelresponse as shown in the right portion of FIG. 4, for example. The APcan filter out the CFO interference by applying a smoothing filteracross frequency.

According to one or more example embodiments, different streams or usersmay send out different channel training sequences across frequency.Because these training sequences may be different, the crosstalk amongthem varies across frequency. There are multiple ways to introduce sucha perturbation in frequency. For example, different streams can usedifferent LTF sequences across frequency. According to one exampleembodiment, multiple LTF sequences may be defined in the standard andthe AP may assign the sequences to different users in uplink MU-MIMO.The LTF sequences may not be completely orthogonal with each other as awhole or as segment by segment because LTF sequence may be constrainedby the requirement of low peak-to-average power ration (PAPR). Accordingto another example embodiment, the orthogonal codes used by the streamscan vary from one subcarrier to the other such that the interaction termbetween code entries i.e. c_(i)*(k) c_(i) (k′) in (6) may vary acrossfrequency. According to yet another embodiment 500, the orthogonal codes502-512 used by the streams may be cyclically shifted from onesubcarrier to the other such that the coefficient of e^(jiΔω) ^(k′) ^(T)in (6) may vary across frequency, as illustrated in FIG. 5, for example.

For maximizing the reuse of the legacy P-matrix, in addition to theP-matrix encoding 604 in time, example systems, methods, and devices mayapply another P-matrix encoding 602 across frequency as shown in theexample embodiment 600 illustrated in FIG. 6, for example. Thetransmitted signal on subcarrier m for stream 606-610 (or user) k andOFDMA symbol i can be written asx _(m,i) =d _(i) _(m) (k)c _(i)(k)s _(m)  (7)

where d_(i) _(m) (k) is the entry at the i_(m)-th row and k-th column of(orthogonal) matrix P_(D) 602 applied across frequency; c_(i) (k) is theentry at the k-th row and i-th column of (orthogonal) matrix P_(C) 604applied across time; M is the number of rows 608 of matrix P_(D) 602.The index i_(m) steps through 1, 2, . . . , M as m increases 1, 2, 3, .. . . There are multiple functions to generate the index i_(m) from m.For example, i_(m)=(m mod M)+1 and i_(m)=((m−1)mod M)+1 are two of them.If the indexes start from 0 instead of 1, then the index i_(m) stepsthrough 0, 1, 2, . . . , M−1 as m increases 0, 1, 2, 3, . . . . Thegenerating function of index i_(m) from m may then be i_(m)=m mod M. Thelegacy 802.11n/ac only have the term c_(i)(k) for time encoding and theterm d_(i) _(m) (k) can be added for frequency encoding, as illustratedin FIG. 6, for example. For the ease of implementation and high reuse,the rows of the 8×8 P-matrix defined in 802.11ac may be used as theorthogonal codes applied across frequency. Namely, P_(D) 602 in FIG. 6is the transpose of the 8×8 P-matrix of 802.11ac or the 8×8 P-matrixitself and M equals 8.

Turning now to FIG. 7, since 802.11ax may support OFDMA, the frequencyband 708 may be divided into multiple sub-bands or sub-channels orresource units (RUs) 704, 706. Each device may be allocated one RU infrequency 708. The application of the frequency domain orthogonal codemay be per resource unit (RU) based or simply for the whole band. Forexample, the code may start from the first entry for each RU in thefirst case 710 and the code entry may be cyclically contiguous over thewhole band for the second case 702 as illustrated in FIG. 7, forexample. Since different RUs 704, 706 may have different numbers ofstreams or users, different matrixes P_(D) with different numbers ofrows and columns may be used for different RUs. For example, the firstresource unit 704 may have three streams and 4×4 orthogonal matrix maybe used, the second resource unit 706 have 6 streams and 8×8 orthogonalmatrix may be used. However, for simplicity, a single orthogonal matrixmay be used for all numbers of streams or users. Since the 8×8 P-matrixof 802.11ac is a combination of 4×4 P-matrix and the 4×4 P-matrix is acombination of 2×2 P-matrix, the 8×8 P-matrix may be used as the singlematrix for all number streams up to 8 streams.

At the AP, the receiver may apply three terms on the N received LTFsymbols on subcarrier m for the channel estimation of stream k, whichare c_(i)*(k) to remove the encoding across time, d_(i) _(m) *(k) toremove the encoding across frequency, and s_(m)* to remove the LFTsequence common to all streams or users. After the effects of codes andLFT sequence are removed, the received signals across time may be summedup together to remove the P-matrix encoding across time. After theP-matrix across time is removed, the received signals are passed to asmoothing filter e.g. low pass filter for removing the interference dueto the CFOs as shown in equation (6), for example.

For simplicity, it can be assumed that the channel response remains thesame across frequency over contiguous M subcarriers, e.g., 2 or 4 or 8.The AP receiver may apply three terms on the N received LTF symbols onsubcarrier m and sums them up for estimating the channel of stream k. Asa result, the interference from the other streams due to the CFOs onsubcarrier m can be rewritten asσ=∥s _(m)∥²Σ_(k′≠k) d _(i) _(m) *(k)d _(i) _(m) (k′)h(k′)Σ_(i=1) ^(N) e^(jiΔω) ^(k′) ^(T) c _(i)*(k)c _(i)(k′)  (8)

Assuming the LTF sequence has a constant modulus that is common in allexisting LTF designs, ∥s_(m)∥² is a constant q. Example systems,methods, and devices can apply a simple, low pass filter with M onesi.e. (1, 1, . . . , 1) to smooth the channel estimation over frequency.At the output of the smoothing filter, the interference due to the CFOsdiminishes as

$\begin{matrix}\begin{matrix}{{\sum\limits_{m = 1}^{M}\vartheta_{m}} = {q{\sum\limits_{m = 1}^{M}{\sum\limits_{k^{\prime} \neq k}{{d_{i_{m}}^{*}(k)}{d_{i_{m}}\left( k^{\prime} \right)}{h\left( k^{\prime} \right)}{\sum\limits_{i = 1}^{N}{{\mathbb{e}}^{{j\mathbb{i}\Delta\omega}_{k^{\prime}}T}{c_{i}^{*}(k)}{c_{i}\left( k^{\prime} \right)}}}}}}}} \\{= {q{\sum\limits_{m = 1}^{M}{\sum\limits_{k^{\prime} \neq k}{{d_{i_{m}}^{*}(k)}{d_{i_{m}}\left( k^{\prime} \right)}{g\left( k^{\prime} \right)}}}}}} \\{= {q{\sum\limits_{k^{\prime} \neq k}{{g\left( k^{\prime} \right)}{\sum\limits_{m = 1}^{M}{{d_{i_{m}}^{*}(k)}{d_{i_{m}}\left( k^{\prime} \right)}}}}}}} \\{= {q{\sum\limits_{k^{\prime} \neq k}{{g\left( k^{\prime} \right)} \cdot 0}}}} \\{= 0}\end{matrix} & (9)\end{matrix}$

where g(k′)=h(k′)Σ_(i=1) ^(N)e^(jiΔω) ^(k′) ^(T)c_(i)*(k)c_(i)(k′); andΣ_(m=1) ^(M)d_(i) _(m) *(k)d_(i) _(m) (k′)=0 is due to the orthogonalityof the codes defined in matrix P_(D) and applied across frequency.

One or more example embodiments may relate to a method for mitigatinginterference in uplink multiuser MIMO. According to this illustrativemethod, an AP receiver may estimate arrival times of each stream oruser's signal using LTF symbols. Since each stream or user sounds thechannel using different sequences (e.g. across frequency) in the LFTsymbols, the AP receiver can estimate the arrival times. The AP may usethe transmitted sequences in time domain as the reference signals,respectively, to match against the received, superimposed signals intime domain. The peaks in the output of this matched filtering indicatethe arrival times.

According to one example embodiment, for streams or users, the differentarrival times causes different linear phase shifts in the frequencydomain channel responses, respectively. Over a group of adjacentsubcarriers in frequency domain, the channel response of each stream oruser can be approximated as a constant complex number with a linearphase shift over frequency. Using the estimated linear shifts and thefrequency domain transmitted codes of the streams or users, the AP cancompute a zero-forcing filter or MMSE filter to mitigate the multiuserinterferences among the streams or users and estimate the complexchannel responses of each stream or user.

According to another example embodiment, for each LTF symbol, the APreceiver obtains the rough channel estimates of each stream or user foreach subcarrier group. For the same subcarrier group, the channelresponses over two different LTF symbols can be compared and the phasedifference can be used to estimate the CFO. The number of subcarriers inthe subcarrier group may be the length of the frequency domain code. TheCFO of each stream can be computed by averaging the CFO estimatesobtained from the subcarrier groups. The estimated CFOs are then used tocompute the linear phase shifts over time over each stream's LTF symbolsequence. For each subcarrier, the linear phase shift is added to eachstream or user's code sequence over time for decoding the P-matrix intime domain. For each subcarrier, using the phase compensated codesequences, the AP can compute a zero-forcing filter or MMSE filter tomitigate the multiuser interference due to CFO for estimating the finechannel responses of each streams or users.

The user specific information may include, for example, user ID(association ID (AID) or partial AID), modulation and coding scheme(MCS), spatial stream indexes, channel coding type (e.g. low densityparity check (LDPC) or binary convolutional coding (BCC)), diversityscheme type (e.g. space-time block coding (STBC) or cyclic shiftdiversity (CSD) or cyclic delay diversity (CDD)) for the user, forexample.

FIG. 8, for example, illustrates example operations that may be involvedin a method 800 for mitigating interference in a Wi-Fi network,according to one or example embodiments. The method 800 may include atblock 802 receiving, by a wireless communication device, one or moredata streams including one or more encoded long training field (LTF)symbols over a wireless communication channel. At block 804, the methodmay include determining, by the wireless communication device, a firstphase of the wireless communication channel upon receipt of a first LTFsymbol, determining, by the wireless communication device, a secondphase of the wireless communication channel upon receipt of a second LTFsymbol, determining, by the wireless communication device, a phasedifference between the first phase and the second phase. At block 806,the method may include determining, by the wireless communicationdevice, a CFO of the wireless communication channel using the determinedphase difference, and modifying, by the wireless communication device,the wireless communication channel estimate based at least in part onthe determined CFO. The method may also include computing, by thewireless communication device, a zero-forcing filter or a minimum meansquare error estimation (MMSE) filter for mitigating interference due tochannel frequency offsets (CFOs) between two or more data streams, anddetermining a channel response of the one or more data streams. Thefirst LTF symbol is a first LTF symbol in a sequence of LTF symbolsincluded in the one or more data streams, and wherein the second LTFsymbol is a later LTF symbol in the same sequence of LTF symbolsincluded in the one or more data streams. The method may also includedetermining, by the wireless communication device, a number of LTFsymbols included in the one or more data streams based on a number ofdata streams determined to be included in the wireless communicationchannel. The wireless communication channel includes one or moresubbands, and wherein each subband is configured to transmit one or moredata streams including the determined number of LTF symbols. The methodmay also include encoding, by the wireless communication device, the oneor more data streams with the one or more LTF symbols using anorthogonal matrix, wherein the dimensions of the orthogonal matrix aredefined by a number of data streams included in the one or more datastreams and a number of LTF symbols included in the one or more LTFsymbols, decoding, by the wireless communication device, the one or moredata streams using the orthogonal matrix, and extracting, by thewireless communication device, the one or more LTF symbols from the oneor more data streams. The method may also include determining, by thewireless communication device, the number of data streams included inthe one or more data streams is less than the number of LTF symbolsincluded in the one or more LTF symbols, and adding one or moreadditional LTF symbols to the one or more LTF symbols to be encoded inthe one or more data streams so that a total number of LTF symbolsencoded in the one or more data streams is equal to the determinednumber of data streams included in the one or more data streams, whereinthe total number of LTF symbols includes a first decodable set of LTFsymbols and a second decodable set of LTF symbols, wherein one or moreadditional LTF symbols are added to the first decodable set using anorthogonal matrix of a first size and one or more additional LTF symbolsare added to the second decodable set using an orthogonal matrix of asecond size different from the first size. FIG. 9, for example,illustrates example operations that may be involved in a method 900 formitigating interference in a Wi-Fi network, according to one or exampleembodiments. The method may include determining a number of LTF symbolsincluded in the one or more data streams based on a number of datastreams determined to be included in the wireless communication channelat block 902. The method 900 may also include at block 904 encoding, bya device including at least one memory including computer-executableinstructions stored thereon, and one or more processing elements toexecute the computer-executable instructions, one or more LTF symbols ina time and/or frequency domain into one or more data streams of awireless communication channel. At block 906 the method may includetransmitting, by the device, the one or more data streams including oneor more encoded LTF symbols over the wireless communication channel. Thewireless communication channel includes one or more subbands, andwherein each subband is configured to transmit one or more data streamsincluding the determined number of LTF symbols.

FIG. 10 shows a functional diagram of an exemplary communication station1000 in accordance with some embodiments. In one embodiment, FIG. 10illustrates a functional block diagram of a communication station thatmay be suitable for use as an AP 102 (FIG. 1) or communication stationSTA 104 (FIG. 1) in accordance with some embodiments. The communicationstation 1000 may also be suitable for use as a handheld device, mobiledevice, cellular telephone, smartphone, tablet, netbook, wirelessterminal, laptop computer, wearable computer device, femtocell, HighData Rate (HDR) subscriber station, access point, access terminal, orother personal communication system (PCS) device.

The communication station 1000 may include physical layer circuitry 1002having a transceiver 1010 for transmitting and receiving signals to andfrom other communication stations using one or more antennas 1020. Thephysical layer circuitry 1002 may also include medium access control(MAC) circuitry 1004 for controlling access to the wireless medium. Thecommunication station 1000 may also include processing circuitry 1006and memory 10010 arranged to perform the operations described herein. Insome embodiments, the physical layer circuitry 1002 and the processingcircuitry 1006 may be configured to perform operations detailed in FIGS.2-9.

In accordance with some embodiments, the MAC circuitry 1004 may bearranged to contend for a wireless medium and configure frames orpackets for communicating over the wireless medium and the physicallayer circuitry 1002 may be arranged to transmit and receive signals.The physical layer circuitry 1002 may include circuitry formodulation/demodulation, upconversion/downconversion, filtering,amplification, etc. In some embodiments, the processing circuitry 1006of the communication station 1000 may include one or more processors. Inother embodiments, two or more antennas 1020 may be coupled to thephysical layer circuitry 1002 arranged for sending and receivingsignals. The memory 1008 may store information for configuring theprocessing circuitry 1006 to perform operations for configuring andtransmitting message frames and performing the various operationsdescribed herein. The memory 1008 may include any type of memory,including non-transitory memory, for storing information in a formreadable by a machine (e.g., a computer). For example, the memory 1008may include a computer-readable storage device may, read-only memory(ROM), random-access memory (RAM), magnetic disk storage media, opticalstorage media, flash-memory devices and other storage devices and media.

In some embodiments, the communication station 1000 may be part of aportable wireless communication device, such as a personal digitalassistant (PDA), a laptop or portable computer with wirelesscommunication capability, a web tablet, a wireless telephone, asmartphone, a wireless headset, a pager, an instant messaging device, adigital camera, an access point, a television, a medical device (e.g., aheart rate monitor, a blood pressure monitor, etc.), a wearable computerdevice, or another device that may receive and/or transmit informationwirelessly.

In some embodiments, the communication station 1000 may include one ormore antennas 1020. The antennas 1020 may include one or moredirectional or omnidirectional antennas, including, for example, dipoleantennas, monopole antennas, patch antennas, loop antennas, microstripantennas or other types of antennas suitable for transmission of RFsignals. In some embodiments, instead of two or more antennas, a singleantenna with multiple apertures may be used. In these embodiments, eachaperture may be considered a separate antenna. In some multiple-inputmultiple-output (MIMO) embodiments, the antennas may be effectivelyseparated for spatial diversity and the different channelcharacteristics that may result between each of the antennas and theantennas of a transmitting station.

In some embodiments, the communication station 1000 may include one ormore of a keyboard, a display, a non-volatile memory port, multipleantennas, a graphics processor, an application processor, speakers, andother mobile device elements. The display may be an LCD screen includinga touch screen.

Although the communication station 1000 is illustrated as having severalseparate functional elements, two or more of the functional elements maybe combined and may be implemented by combinations ofsoftware-configured elements, such as processing elements includingdigital signal processors (DSPs), and/or other hardware elements. Forexample, some elements may include one or more microprocessors, DSPs,field-programmable gate arrays (FPGAs), application specific integratedcircuits (ASICs), radio-frequency integrated circuits (RFICs) andcombinations of various hardware and logic circuitry for performing atleast the functions described herein. In some embodiments, thefunctional elements of the communication station 1000 may refer to oneor more processes operating on one or more processing elements.

Certain embodiments may be implemented in one or a combination ofhardware, firmware and software. Other embodiments may also beimplemented as instructions stored on a computer-readable storagedevice, which may be read and executed by at least one processor toperform the operations described herein. The instructions may be in anysuitable form, such as but not limited to source code, compiled code,interpreted code, executable code, static code, dynamic code, and thelike. A computer-readable storage device or medium may include anynon-transitory memory mechanism for storing information in a formreadable by a machine (e.g., a computer). For example, acomputer-readable storage device may include read-only memory (ROM),random-access memory (RAM), magnetic disk storage media, optical storagemedia, flash-memory devices, and other storage devices and media. Insome embodiments, the communication station 1000 may include one or moreprocessors and may be configured with instructions stored on acomputer-readable storage device memory.

FIG. 11 illustrates a block diagram of an example of a machine 1100 orsystem upon which any one or more of the techniques (e.g.,methodologies) discussed herein may be performed. In other embodiments,the machine 1100 may operate as a standalone device or may be connected(e.g., networked) to other machines. In a networked deployment, themachine 1100 may operate in the capacity of a server machine, a clientmachine, or both in server-client network environments. In an example,the machine 1100 may act as a peer machine in peer-to-peer (P2P) (orother distributed) network environment. The machine 1100 may be apersonal computer (PC), a tablet PC, a set-top box (STB), a personaldigital assistant (PDA), a mobile telephone, wearable computer device, aweb appliance, a network router, switch or bridge, or any machinecapable of executing instructions (sequential or otherwise) that specifyactions to be taken by that machine, such as a base station. Further,while only a single machine is illustrated, the term “machine” shallalso be taken to include any collection of machines that individually orjointly execute a set (or multiple sets) of instructions to perform anyone or more of the methodologies discussed herein, such as cloudcomputing, software as a service (SaaS), or other computer clusterconfigurations.

Examples, as described herein, may include, or may operate on, logic ora number of components, modules, or mechanisms. Modules are tangibleentities (e.g., hardware) capable of performing specified operationswhen operating. A module includes hardware. In an example, the hardwaremay be specifically configured to carry out a specific operation (e.g.,hardwired). In another example, the hardware may include configurableexecution units (e.g., transistors, circuits, etc.) and a computerreadable medium containing instructions, where the instructionsconfigure the execution units to carry out a specific operation when inoperation. The configuring may occur under the direction of theexecutions units or a loading mechanism. Accordingly, the executionunits are communicatively coupled to the computer readable medium whenthe device is operating. In this example, the execution units may be amember of more than one module. For example, under operation, theexecution units may be configured by a first set of instructions toimplement a first module at one point in time and reconfigured by asecond set of instructions to implement a second module at a secondpoint in time.

The machine (e.g., computer system) 1100 may include a hardwareprocessor 1102 (e.g., a central processing unit (CPU), a graphicsprocessing unit (GPU), a hardware processor core, or any combinationthereof), a main memory 1104 and a static memory 1106, some or all ofwhich may communicate with each other via an interlink (e.g., bus) 1108.The machine 1100 may further include a power management device 1132, agraphics display device 1110, an alphanumeric input device 1112 (e.g., akeyboard), and a user interface (UI) navigation device 1114 (e.g., amouse). In an example, the graphics display device 1110, alphanumericinput device 1112 and UI navigation device 1114 may be a touch screendisplay. The machine 1100 may additionally include a storage device(i.e., drive unit) 1116, a signal generation device 1118 (e.g., aspeaker), a network interface device/transceiver 1120 coupled toantenna(s) 1130, and one or more sensors 1128, such as a globalpositioning system (GPS) sensor, compass, accelerometer, or othersensor. The machine 1100 may include an output controller 1134, such asa serial (e.g., universal serial bus (USB), parallel, or other wired orwireless (e.g., infrared (IR), near field communication (NFC), etc.)connection to communicate with or control one or more peripheral devices(e.g., a printer, card reader, etc.)

The storage device 1116 may include a machine readable medium 1122 onwhich is stored one or more sets of data structures or instructions 1124(e.g., software) embodying or utilized by any one or more of thetechniques or functions described herein. The instructions 1124 may alsoreside, completely or at least partially, within the main memory 1104,within the static memory 1106, or within the hardware processor 1102during execution thereof by the machine 1100. In an example, one or anycombination of the hardware processor 1102, the main memory 1104, thestatic memory 1106, or the storage device 1116 may constitute machinereadable media.

While the machine readable medium 1122 is illustrated as a singlemedium, the term “machine readable medium” may include a single mediumor multiple media (e.g., a centralized or distributed database, and/orassociated caches and servers) configured to store the one or moreinstructions 1124.

The term “machine readable medium” may include any medium that iscapable of storing, encoding, or carrying instructions for execution bythe machine 1100 and that cause the machine 1100 to perform any one ormore of the techniques of the present disclosure, or that is capable ofstoring, encoding or carrying data structures used by or associated withsuch instructions. Non-limiting machine readable medium examples mayinclude solid-state memories, and optical and magnetic media. In anexample, a massed machine readable medium includes a machine readablemedium with a plurality of particles having resting mass. Specificexamples of massed machine readable media may include: non-volatilememory, such as semiconductor memory devices (e.g., ElectricallyProgrammable Read-Only Memory (EPROM), or Electrically ErasableProgrammable Read-Only Memory (EEPROM)) and flash memory devices;magnetic disks, such as internal hard disks and removable disks;magneto-optical disks; and CD-ROM and DVD-ROM disks.

The instructions 1124 may further be transmitted or received over acommunications network 1126 using a transmission medium via the networkinterface device/transceiver 1120 utilizing any one of a number oftransfer protocols (e.g., frame relay, internet protocol (IP),transmission control protocol (TCP), user datagram protocol (UDP),hypertext transfer protocol (HTTP), etc.). Example communicationsnetworks may include a local area network (LAN), a wide area network(WAN), a packet data network (e.g., the Internet), mobile telephonenetworks (e.g., cellular networks), Plain Old Telephone (POTS) networks,wireless data networks (e.g., Institute of Electrical and ElectronicsEngineers (IEEE) 802.11 family of standards known as Wi-Fi®, IEEE 802.16family of standards known as WiMax®), IEEE 802.15.4 family of standards,and peer-to-peer (P2P) networks, among others. In an example, thenetwork interface device/transceiver 1120 may include one or morephysical jacks (e.g., Ethernet, coaxial, or phone jacks) or one or moreantennas to connect to the communications network 1126. In an example,the network interface device/transceiver 1120 may include a plurality ofantennas to wirelessly communicate using at least one of single-inputmultiple-output (SIMO), multiple-input multiple-output (MIMO), ormultiple-input single-output (MISO) techniques. The term “transmissionmedium” shall be taken to include any intangible medium that is capableof storing, encoding or carrying instructions for execution by themachine 1100, and includes digital or analog communications signals orother intangible media to facilitate communication of such software.

In one embodiment, machine 1100 can include a set of one or moretransmitters/receivers 1120, and components therein (amplifiers,filters, analog-to-digital (A/D) converters, etc.), functionally coupledto a multiplexer/demultiplexer (mux/demux) unit, a modulator/demodulator(mod/demod) unit (also referred to as a modem), and an encoder/decoderunit (also referred to as codec). Each of the transmitter(s)/receiver(s)can form respective transceiver(s) that can transmit and receivewireless signal (e.g., streams, electromagnetic radiation) via the oneor more antennas.

Electronic components and associated circuitry, such as mux/demux unit,codec, and modem can permit or facilitate processing and manipulation,e.g., coding/decoding, deciphering, and/or modulation/demodulation, ofsignal(s) received by the computing device and signal(s) to betransmitted by the computing device. In one aspect, as described herein,received and transmitted wireless signals can be modulated and/or coded,or otherwise processed, in accordance with one or more radio technologyprotocols. Such radio technology protocol(s) can include 3GPP UMTS; 3GPPLTE; LTE-A; Wi-Fi protocols, such as IEEE 802.11 family of standards(IEEE 802.ac, IEEE 802.ax, and the like); WiMAX; radio technologies andrelated protocols for ad hoc networks, such as Bluetooth or ZigBee;other protocols for packetized wireless communication; or the like.

The electronic components in the described communication unit, includingthe one or more transmitters/receivers 1120, can exchange information(e.g., streams, LTF symbols, data, metadata, code instructions,signaling and related payload data, combinations thereof, or the like)through a bus, which can embody or can comprise at least one of a systembus, an address bus, a data bus, a message bus, a reference link orinterface, a combination thereof, or the like. Each of the one or morereceivers/transmitters can convert signal from analog to digital andvice versa. In addition or in the alternative, thereceiver(s)/transmitter(s) can divide a single data stream into multipleparallel data streams, or perform the reciprocal operation. Suchoperations may be conducted as part of various multiplexing schemes. Asillustrated, the mux/demux unit is functionally coupled to the one ormore receivers/transmitters and can permit processing of signals in timeand frequency domain. In one aspect, the mux/demux unit can multiplexand demultiplex information (e.g., data, metadata, and/or signaling)according to various multiplexing schemes such as time divisionmultiplexing (TDM), frequency division multiplexing (FDM), orthogonalfrequency division multiplexing (OFDM), code division multiplexing(CDM), space division multiplexing (SDM). In addition or in thealternative, in another aspect, the mux/demux unit can scramble andspread information (e.g., codes) according to most any code, such asHadamard-Walsh codes, Baker codes, Kasami codes, polyphase codes, andthe like. The modem can modulate and demodulate information (e.g., data,metadata, signaling, or a combination thereof) according to variousmodulation techniques, such as frequency modulation (e.g.,frequency-shift keying), amplitude modulation (e.g., M-ary quadratureamplitude modulation (QAM), with M a positive integer; amplitude-shiftkeying (ASK)), phase-shift keying (PSK), and the like). In addition,processor(s) that can be included in the computing device (e.g.,processor(s) included in the radio unit or other functional element(s)of the computing device) can permit processing data (e.g., symbols,bits, or chips) for multiplexing/demultiplexing, modulation/demodulation(such as implementing direct and inverse fast Fourier transforms)selection of modulation rates, selection of data packet formats,inter-packet times, and the like.

The codec can operate on information (e.g., data, metadata, signaling,or a combination thereof) in accordance with one or more coding/decodingschemes suitable for communication, at least in part, through the one ormore transceivers formed from respective transmitter(s)/receiver(s). Inone aspect, such coding/decoding schemes, or related procedure(s), canbe retained as a group of one or more computer-accessible instructions(computer-readable instructions, computer-executable instructions, or acombination thereof) in one or more memory devices (referred to asmemory). In a scenario in which wireless communication among thecomputing device and another computing device (e.g., an access point, auser device, a station and/or other types of user equipment) utilizesMU-MIMI, MIMO, MISO, SIMO, or SISO operation, the codec can implement atleast one of space-time block coding (STBC) and associated decoding, orspace-frequency block (SFBC) coding and associated decoding. In additionor in the alternative, the codec can extract information from datastreams coded in accordance with spatial multiplexing scheme. In oneaspect, to decode received information (e.g., data, metadata, signaling,or a combination thereof), the codec can implement at least one ofcomputation of log-likelihood ratios (LLR) associated with constellationrealization for a specific demodulation; maximal ratio combining (MRC)filtering, maximum-likelihood (ML) detection, successive interferencecancellation (SIC) detection, zero forcing (ZF) and minimum mean squareerror estimation (MMSE) detection, or the like. The codec can utilize,at least in part, mux/demux component and mod/demod component to operatein accordance with aspects described herein.

While there have been shown, described and pointed out, fundamentalnovel features of the invention as applied to the exemplary embodimentsthereof, it will be understood that various omissions and substitutionsand changes in the form and details of devices illustrated, and in theiroperation, may be made by those skilled in the art without departingfrom the spirit of the invention. Moreover, it is expressly intendedthat all combinations of those elements and/or method operations, whichperform substantially the same function in substantially the same way toachieve the same results, are within the scope of the disclosure.Moreover, it should be recognized that structures and/or elements and/ormethod operations shown and/or described in connection with anydisclosed form or embodiment of the disclosure may be incorporated inany other disclosed or described or suggested form or embodiment as ageneral matter of design choice. It is the intention, therefore, to belimited only as indicated by the scope of the claims appended hereto.

Example Embodiments

One example embodiment is a computer-readable non-transitory storagemedium that contains computer-executable instructions, which whenexecuted by one or more processors result in performing operationsincluding identifying one or more data streams including one or moreencoded long training field (LTF) symbols over a wireless communicationchannel, determining a first phase of the wireless communication channelupon receipt of a first LTF symbol, determining a second phase of thewireless communication channel upon receipt of a second LTF symbol,determining a phase difference between the first phase and the secondphase, determining a channel frequency offset (CFO) of the wirelesscommunication channel using the phase difference, and determining thewireless communication channel estimate based at least in part on thedetermined CFO. The medium operations further comprise determining azero-forcing filter or a minimum mean square error estimation (MMSE)filter for mitigating interference due to channel frequency offsets(CFOs) between two or more data streams, and determining a channelresponse of the one or more data streams. The first LTF symbol is afirst LTF symbol in a sequence of LTF symbols included in the one ormore data streams, and wherein the second LTF symbol is a last LTFsymbol in a second sequence of LTF symbols included in the one or moredata streams. The medium operations further comprise determining anumber of LTF symbols included in the one or more data streams based ona number of data streams determined to be included in the wirelesscommunication channel. The wireless communication channel includes oneor more subbands, and wherein each subband is configured to transmit oneor more data streams including the determined number of LTF symbols. Themedium operations further comprise encoding the one or more data streamswith the one or more LTF symbols using an orthogonal matrix, wherein thedimensions of the orthogonal matrix are defined by a number of datastreams included in the one or more data streams and a number of LTFsymbols included in the one or more LTF symbols, decode the one or moredata streams using the orthogonal matrix, and extract the one or moreLTF symbols from the one or more data streams. The medium operationsfurther comprise determining the number of data streams included in theone or more data streams is less than the number of LTF symbols includedin the one or more LTF symbols, and adding one or more additional LTFsymbols to the one or more LTF symbols to be encoded in the one or moredata streams so that a total number of LTF symbols encoded in the one ormore data streams is equal to the determined number of data streamsincluded in the one or more data streams, wherein the total number ofLTF symbols includes a first decodable set of LTF symbols and a seconddecodable set of LTF symbols.

One example embodiment is a method for mitigating interference in awireless network. The method may include receiving, by a wirelesscommunication device, one or more data streams including one or moreencoded long training field (LTF) symbols over a wireless communicationchannel, determining, by the wireless communication device, a firstphase of the wireless communication channel upon receipt of a first LTFsymbol, determining, by the wireless communication device, a secondphase of the wireless communication channel upon receipt of a second LTFsymbol, determining, by the wireless communication device, a phasedifference between the first phase and the second phase, determining, bythe wireless communication device, a CFO of the wireless communicationchannel using the determined phase difference, and modifying, by thewireless communication device, the wireless communication channelestimate based at least in part on the determined CFO. The method mayalso include computing, by the wireless communication device, azero-forcing filter or a minimum mean square error estimation (MMSE)filter for mitigating interference due to channel frequency offsets(CFOs) between two or more data streams, and determining a channelresponse of the one or more data streams. The first LTF symbol is afirst LTF symbol in a sequence of LTF symbols included in the one ormore data streams, and wherein the second LTF symbol is a last LTFsymbol in a second sequence of LTF symbols included in the one or moredata streams. The method may also include determining, by the wirelesscommunication device, a number of LTF symbols included in the one ormore data streams based on a number of data streams determined to beincluded in the wireless communication channel. The wirelesscommunication channel includes one or more subbands, and wherein eachsubband is configured to transmit one or more data streams including thedetermined number of LTF symbols. The method may also include encoding,by the wireless communication device, the one or more data streams withthe one or more LTF symbols using an orthogonal matrix, wherein thedimensions of the orthogonal matrix are defined by a number of datastreams included in the one or more data streams and a number of LTFsymbols included in the one or more LTF symbols, decoding, by thewireless communication device, the one or more data streams using theorthogonal matrix, and extracting, by the wireless communication device,the one or more LTF symbols from the one or more data streams. Themethod may also include determining, by the wireless communicationdevice, the number of data streams included in the one or more datastreams is less than the number of LTF symbols included in the one ormore LTF symbols, and adding one or more additional LTF symbols to theone or more LTF symbols to be encoded in the one or more data streams sothat a total number of LTF symbols encoded in the one or more datastreams is equal to the determined number of data streams included inthe one or more data streams, wherein the total number of LTF symbolsincludes a first decodable set of LTF symbols and a second decodable setof LTF symbols, wherein one or more additional LTF symbols are added tothe first decodable set using an orthogonal matrix of a first size andone or more additional LTF symbols are added to the second decodable setusing an orthogonal matrix of a second size different from the firstsize.

One example embodiment is a device including at least one memoryincluding computer-executable instructions stored thereon, and one ormore processing elements to execute the computer-executable instructionsto encode one or more long training field (LTF) symbols, in a timeand/or frequency domain, into one or more data streams of a wirelesscommunication channel, and transmit the one or more data streamsincluding one or more encoded LTF symbols over the wirelesscommunication channel. The device may be configured to determine anumber of LTF symbols included in the one or more data streams based ona number of data streams determined to be included in the wirelesscommunication channel. The wireless communication channel includes oneor more subbands, and wherein each subband is configured to transmit oneor more data streams including the determined number of LTF symbols.

A computer-readable non-transitory storage medium that containscomputer-executable instructions, which when executed by one or moreprocessors result in performing operations including encoding, by adevice including at least one memory including computer-executableinstructions stored thereon, and one or more processing elements toexecute the computer-executable instructions, one or more LTF symbols ina time and/or frequency domain into one or more data streams of awireless communication channel, and transmitting, by the device, the oneor more data streams including one or more encoded LTF symbols over thewireless communication channel. The medium may also include determininga number of LTF symbols included in the one or more data streams basedon a number of data streams determined to be included in the wirelesscommunication channel. The wireless communication channel includes oneor more subbands, and wherein each subband is configured to transmit oneor more data streams including the determined number of LTF symbols.

What is claimed is:
 1. A computer-readable non-transitory storage mediumthat contains computer-executable instructions, which when executed byone or more processors result in performing operations comprising:identifying one or more data streams comprising one or more encoded longtraining field (LTF) symbols over a wireless communication channel;encoding the one or more data streams comprising the one or more LTFsymbols using an orthogonal matrix, wherein dimensions of the orthogonalmatrix are determined by a number of data streams comprised in the oneor more data streams and a number of LTF symbols comprised in the one ormore LTF symbols; determining a first phase of the wirelesscommunication channel using a first LTF symbol of the one or more LTFsymbols; determining a second phase of the wireless communicationchannel using a second LTF symbol of the one or more LTF symbols;determining a phase difference between the first phase and the secondphase; determining a channel frequency offset (CFO) of the wirelesscommunication channel using the phase difference; and determining awireless communication channel response based at least in part on thedetermined CFO.
 2. The medium of claim 1, wherein the operations furthercomprise: determining a zero-forcing filter or a minimum mean squareerror estimation (MMSE) filter for mitigating interference on thewireless communication channel due to channel frequency offsets (CFOs)between two or more data streams of the one or more data streams; anddetermining a channel response of the one or more data streams.
 3. Themedium of claim 1, wherein the first LTF symbol is a first LTF symbol ina sequence of LTF symbols comprised in the one or more data streams, andwherein the second LTF symbol is a later LTF symbol in the same sequenceof LTF symbols comprised in the one or more data streams.
 4. The mediumof claim 1, wherein the operations further comprise: determining anumber of LTF symbols comprised in the one or more data streams based ona number of data streams determined to be comprised in the wirelesscommunication channel.
 5. The medium of claim 4, wherein the wirelesscommunication channel comprises one or more subbands, and wherein eachsubband is configured to transmit the one or more data streamscomprising the determined number of LTF symbols.
 6. The medium of claim1, wherein the operations further comprise: determining the number ofdata streams comprised in the one or more data streams is less than thenumber of LTF symbols comprised in the one or more LTF symbols; andadding one or more additional LTF symbols to the one or more LTF symbolsto be encoded in the one or more data streams so that a total number ofLTF symbols encoded in the one or more data streams is equal to thedetermined number of data streams comprised in the one or more datastreams, wherein the total number of LTF symbols comprises a firstdecodable set of LTF symbols and a second decodable set of LTF symbols.7. A method for mitigating interference in a wireless network, themethod comprising: identifying, by a wireless communication device, oneor more data streams comprising one or more encoded long training field(LTF) symbols over a wireless communication channel; encoding the one ormore data streams comprising the one or more LTF symbols using anorthogonal matrix, wherein dimensions of the orthogonal matrix aredetermined by a number of data streams comprised in the one or more datastreams and a number of LTF symbols comprised in the one or more LTFsymbols; determining, by the wireless communication device, a firstphase of the wireless communication channel using a first LTF symbol ofthe one or more LTF symbols; determining, by the wireless communicationdevice, a second phase of the wireless communication channel using asecond LTF symbol of the one or more LTF symbols; determining, by thewireless communication device, a phase difference between the firstphase and the second phase; determining, by the wireless communicationdevice, a CFO of the wireless communication channel using the determinedphase difference; and determining, by the wireless communication device,the wireless communication channel response based at least in part onthe determined CFO.
 8. The method of claim 7, further comprising:determining, by the wireless communication device, a zero-forcing filteror a minimum mean square error estimation (MMSE) filter for mitigatinginterference due to channel frequency offsets (CFOs) between two or moredata streams; and determining a channel response of the one or more datastreams.
 9. The method of claim 7, wherein the first LTF symbol is afirst LTF symbol in a sequence of LTF symbols comprised in the one ormore data streams, and wherein the second LTF symbol is a later LTFsymbol in the same sequence of LTF symbols comprised in the one or moredata streams.
 10. The method of claim 7, wherein the method furthercomprises: determining, by the wireless communication device, a numberof LTF symbols comprised in the one or more data streams based on anumber of data streams determined to be comprised in the wirelesscommunication channel.
 11. The method of claim 10, wherein the wirelesscommunication channel comprises one or more subbands, and wherein eachsubband is configured to transmit one or more data streams comprisingthe determined number of LTF symbols.
 12. The method of claim 7, whereinthe encoding further comprises: determining, by the wirelesscommunication device, the number of data streams comprised in the one ormore data streams is less than the number of LTF symbols comprised inthe one or more LTF symbols; and adding one or more additional LTFsymbols to the one or more LTF symbols to be encoded in the one or moredata streams so that a total number of LTF symbols encoded in the one ormore data streams is equal to the determined number of data streamscomprised in the one or more data streams, wherein the total number ofLTF symbols comprises a first decodable set of LTF symbols and a seconddecodable set of LTF symbols, wherein one or more additional LTF symbolsare added to the first decodable set using an orthogonal matrix of afirst size and one or more additional LTF symbols are added to thesecond decodable set using an orthogonal matrix of a second sizedifferent from the first size.
 13. A device, comprising: at least onememory comprising computer-executable instructions stored thereon; andone or more processing elements to execute the computer-executableinstructions to: determine a number of long training field (LTF) symbolscomprised in one or more data streams based at least in part on a numberof data streams comprised in a wireless communication channel; encodeone or more LTF symbols, in a time and/or frequency domain, into the oneor more data streams of the wireless communication channel, wherein theencoding is performed using an orthogonal matrix, wherein dimensions ofthe orthogonal matrix are determined by the number of data streams andthe number of LTF symbols; and cause to transmit the one or more datastreams comprising the one or more encoded LTF symbols over the wirelesscommunication channel.
 14. The device of claim 13, wherein the device isfurther configured to: determine a number of LTF symbols comprised inthe one or more data streams based on a number of data streamsdetermined to be comprised in the wireless communication channel. 15.The device of claim 14, wherein the wireless communication channelcomprises one or more subbands, and wherein each subband is configuredto transmit one or more data streams comprising the determined number ofLTF symbols.
 16. A computer-readable non-transitory storage medium thatcontains computer-executable instructions, which when executed by one ormore processors result in performing operations comprising: determining,by a wireless communication device, a number of long training field(LTF) symbols comprised in one or more data streams based at least inpart on a number of data streams comprised in a wireless communicationchannel; encoding, by the wireless communication device, the one or moreLTF symbols in a time and/or frequency domain into the one or more datastreams of the wireless communication channel, wherein the encoding isperformed using an orthogonal matrix, wherein dimensions of theorthogonal matrix are determined by the number of data streams and thenumber of LTF symbols; and causing to transmit, by the wirelesscommunication device, the one or more data streams comprising the one ormore encoded LTF symbols over the wireless communication channel. 17.The medium of claim 16, wherein the operations further comprise:determining a number of LTF symbols comprised in the one or more datastreams based on an indication from a multiuser receiver device.
 18. Themedium of claim 16, wherein the wireless communication channel comprisesone or more subbands, and wherein each subband is configured to transmitone or more data streams comprising the determined number of LTFsymbols.